This is something I've been dancing around for years, never sure. A LIU shows that there are two separate words, sherbet and sorbet, the former having the variant sherbert. Each traces back to the turkish word sherbet.

Says bartleby at http://www.bartleby.com/61/75/S0337500.html:
sherbet NOUN: 1. also sher·bert ... A frozen dessert [etc.]
... Sherbet came into English from Ottoman Turkish sherbet or Persian sharbat, ... Because the original Middle Eastern drink contained fruit and was often cooled with snow, sherbet was applied to a frozen dessert (first recorded in 1891). It is distinguished slightly from sorbet, which can also mean "a fruit-flavored ice served between courses of a meal." Sorbet ... goes back through French (sorbet) and then Italian (sorbetto) to the same Turkish sherbet that gave us sherbet.


PS: I'm hungry. Durn these food-discussions.